Doors for railway transit vehicles must be designed to function rapidly and safely without the direct observation thereof by an operator of the railway transit vehicle. Such doors generally receive signals from trainlines which direct them to open and close. The trainlines usually are multistrand electrical communication cables which are connected from car to car down the length of a train. Such doors generally have sensors which provide signals which indicate whether they are open or closed, and those signals are transmitted via the trainlines back to a control system for the train.
The design of transit doors is crucial for the safety of the travelling public, and they function in a hostile environment which includes heavy usage, temperature extremes, vibration and acceleration loads, and spurious electrical signals which may be caused by lightning, by interrupted contact with a third rail, or strong radio signals.
It is therefore highly desirable, and in many jurisdictions required, for the doors and their control system to have a number of safety features. These include:
(1) The doors should have locks so that they cannot be opened by application of opening forces to the doors when the doors are closed, without energization of the motor which is used to open the doors. PA1 (2) The doors should have sensors which indicate whether or not they are locked in the closed position. PA1 (3) No credible failure mode should cause a door to open sufficiently, when the train is moving, for a person to fall out of the railway vehicle. PA1 (4) The doors should be interlocked with the control system of the train so that the train cannot be put in motion until the doors are closed sufficiently that a person cannot fall out of the railway vehicle. PA1 (5) The doors should be interlocked with the control system of the train so that the train cannot exceed a predetermined speed until all the doors are locked in a fully-closed position.
Generally, prior art systems have various layers of safety systems to accomplish the objectives above. The layers of safety systems required in prior art systems themselves contribute to failure modes. Additionally, some prior systems provide motive power for the lock system from the same source as the door activating motor which moves the doors from closed to open and from open to closed. Hence, a spurious electrical signal to the door activating motor may cause the motor to unlock the doors and move them to an open position.